Carbonized phenanthroline functionalized carbon as an alternative support: A strategy to intensify Pt activity and durability for methanol oxidation

Article abstract of DOI:10.1039/c4ra14790a

In this study, a carbonized phenanthroline functionalized carbon support (CPF-C) has been prepared, which was used to load Pt nanoparticles to serve as an electrocatalyst for methanol oxidation. The results show that the performance of Pt in Pt/CPF-C is improved relative to that of industrially adopted Pt/C. The increase is mainly interpreted in terms of the synergistic effect between the stabilized Pt nanoparticles and nitrogen-functionalized CPF-C support. The novel CPF-C support presented here offers an alternative to the conventional carbon support to improve the performance of Pt applied in direct methanol fuel cells. This journal is

Full text of DOI:10.1039/c4ra14790a

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Carbonized phenanthroline functionalized carbon
as an alternative support: a strategy to intensify Pt
activity and durability for methanol oxidation†
Cite this: RSC Adv., 2015, 5, 17216
ab
*
Wei Wang,
Yan Yang,b Fengxia Wang,b Wenkui Dong,a Xiaozhong Zhoub
*
b
and Ziqiang Lei
In this study, a carbonized phenanthroline functionalized carbon support (CPF-C) has been prepared, which
was used to load Pt nanoparticles to serve as an electrocatalyst for methanol oxidation. The results show
that the performance of Pt in Pt/CPF-C is improved relative to that of industrially adopted Pt/C. The
increase is mainly interpreted in terms of the synergistic effect between the stabilized Pt nanoparticles
and nitrogen-functionalized CPF-C support. The novel CPF-C support presented here offers an
alternative to the conventional carbon support to improve the performance of Pt applied in direct
methanol fuel cells.
Received 18th November 2014
Accepted 2nd February 2015
DOI: 10.1039/c4ra14790a
www.rsc.org/advances
It is reported that nitrogen-doping can enhance electron
transfer properties.29 The nitrogen-doping could not only
1. Introduction
change the physicochemical and electronic properties of the
support, but also serve as basic or coordination sites to stabilize
the small metal nanoparticles or activate some special
substrates.30 Thus, constructing nitrogen-functionalized carbon
material has become one of the most attractive research topics
because it is highly promising as alternative support for cata-
lytic application.31,32 Normally, nitrogen-functionalized carbon
allotropes and their derivatives have been studied as alternative
supports.10,20,21,33–35 However, using phenanthroline as nitrogen
source to synthesize nitrogen-functionalized alternative support
for Pt in methanol oxidation is rarely reported.
Therefore, in this study, a catalyst of Pt nanoparticles on
carbonized phenanthroline functionalized carbon support
(CPF-C) has been prepared, characterized, and evaluated in the
methanol oxidation reaction (MOR). The fabrication process
results in uniformity in the particle size and strong particle-
support interaction, beneting for the performances of Pt/
CPF-C catalyst. The catalyst exhibits high electrocatalytic prop-
erties, including high electrocatalytic activity, and relatively
reliable stability and durability.
Direct methanol fuel cells (DMFCs) have been considered as a
promising alternative to conventional fossil fuels. Traditionally,
Pt-based materials are thought of as the best electrocatalysts.
However, as Pt has a high cost and its reserves are limited,
extensive efforts have focused on promoting Pt performance by
creating advanced novel catalysts. Approaches can be catego-
rized into three types: rstly, formation of multi-component (Pt
alloyed with other metals) or unique structures of Pt-based
nanoparticles (core–shell, porous, sheet, et al.);1–8 secondly,
reducing the size of Pt-based nanoparticles;9–15 lastly, using
novel materials or alternative heteroatom carbon supports.16–25
Undoubtedly, to move towards a genuinely practical DMFC
technology, further improvements are still needed.26
For supports, such as carbon, carbon nanotube and gra-
phene, the effect is to hinder agglomeration of loaded nano-
particles and to afford a high electron conductance between
nanoparticles and the bulk electrode.27 An advanced support
can further affect the activity of the catalyst by an interaction
between the support and nanoparticles. It can modify the
electronic character of the nanoparticles, which would enhance
the performance of the catalyst.28
2. Experimental
2.1 CPF-C support synthesis
aSchool of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou
730070, China. E-mail: wangwchem@163.com; Fax: +86 931 4938755; Tel: +86 931
4938755
bKey Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education
of China, Key Laboratory of Gansu Polymer Materials, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, China.
E-mail: leizq@nwnu.edu.cn; leizq@hotmail.com; Fax: +86 931 7970261; Tel: +86
931 7970261
In a typical approach, phenanthroline (0.50 g), NiCl2$6H2O
(0.24 g) and the pretreated carbon black (1.00 g, Vulcan XC-72R)
were dissolved/dispersed in 40 ml ethanol and stirred for 12 h.
Then, the ethanol was completely removed by a rotary evapo-
rator. The remained solid was dried for 6 h at 80 C and heat-
treated at 800 ꢀC for 2 h in tube furnace in N2 atmosphere.
Nextly, the solid was treated in 0.5 M H2SO4 for 12 h to remove
ꢀ
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14790a
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ꢀ
unstable species, then centrifuged, ltrated and dried at 50 C
for 12 h. The product was named as CPF-C.
2.2 Pt/CPF-C catalyst synthesis
H2PtCl6 (66.4 mg) and sodium citrate (100.0 mg) were dissolved
in 30 ml ethylene glycol (EG) and stirred for l h. The pH value
was adjusted to ꢁ8 by adding 5 wt% KOH/EG solution. Aer-
wards, the CPF-C powder (100.0 mg) was added with stirring
and dispersing in an ultrasonic bath for 0.5 h, respectively.
ꢀ
Then the suspension was heated at 180 C for 4 h, centrifuged
and washed with distilled water. The product was dried for 4 h
at 50 ꢀC. Finally, the Pt/CPF-C catalyst was obtained. For
comparison, the industrially adopted commercial Pt/C was used
as a reference (Johnson Matthey Company, 20 wt% metal
loaded, HiSPEC 3000, 3.5 nm).
Fig. 1 TG spectrum of preparing CPF-C support from the precursor of
phenanthroline chelate–carbon composite.
2.3 Characterization
Particle size and morphology were obtained on a transmission
electron microscope (FEI TECNAI G2 TF20 America). X-ray
diffraction (XRD) patterns were recorded on a Rigaku D/Max-
2400 (Japan) diffractometer, using Cu Ka radiation operated at
40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS) (Thi-
5702 America) was a monochromatic Al Ka X-ray source (hn ¼
29.35 eV). The pyrolysis monitoring was performed on a ther-
mogravimetric analysis (PerkinElmer, TG/DTA 6300, America).
The electrochemical experiments were performed on an
electrochemical workstation (Autolab, PGSTAT128N, Eco
Chemie, Netherlands). A glassy carbon (5 mm in diameter), a Pt
wire and an Ag/AgCl were used as the working, the auxiliary and
the reference electrodes, respectively. The working electrode
was prepared as follows: 5 mg catalyst was dispersed ultrason-
ically in 1 ml Naon/ethanol (0.25% Naon). 8 ml suspensions
were quantitatively transferred to the surface of polished glassy
carbon electrode, followed by dried in air. Before each
measurement, the solution was purged with high-purity N2 for
at least 10 min to ensure N2 saturated.
immobilize and stabilize small Pt nanoparticles, and would
demonstrate high catalytic activity and durability in MOR.
The morphology of as-prepared Pt/CPF-C catalyst is shown in
Fig. 2a. The small Pt nanoparticles are formed by anchoring the
Pt ion on the CPF-C surface and subsequently reducing to the
ne particles, without any signicant agglomeration. Based on
two hundred Pt particles randomly selected, Fig. 2b shows the
corresponded particle size distribution histograms. The parti-
cles are dispersed on the support with narrow size distribution
and the mean size of the Pt particles is about 3.9 nm. The Pt/
CPF-C particles are slightly larger than that of commercial Pt/
C (3.5 nm). However, based on the synergistic effect between
the Pt nanoparticles and CPF-C support, the Pt/CPF-C also
shows the high performance in MOR below. The high-
magnication transmission electron microscope (TEM) image
of an individual Pt nanoparticle of Pt/CPF-C shows its crystal-
line structure in Fig. 2c. The measured interplanar spacing of
the lattice fringes is 0.23 nm, which is the lattice spacing of the
(111) plane in face-centered cubic (fcc) Pt.10,36 As shown in
Fig. 2d, the selected area electron diffraction (SAED) on Pt/CPF-
C catalyst shows concentric rings composed of bright discrete
diffraction spots that can be indexed to Pt (fcc), indicating the
polycrystalline of the Pt nanoparticles.
3. Results and discussion
The CPF-C support was prepared from a phenanthroline
chelate, which is easy to large-scale production. And the
pyrolysis of phenanthroline chelate–carbon composite
precursor was monitored by TG analysis shown in Fig. 1. The
loss of adsorbed water is below 100 ꢀC. The precursor begins to
decompose at 170 ꢀC and desorbs to volatile compounds
between 170 and 450 ꢀC. Then it is relatively stable between
The XPS measurements were performed to probe the
chemical composition of the Pt/CPF-C catalyst. As shown in
Fig. 3a, the XPS spectrum of Pt/CPF-C catalyst shows the pres-
ence of N 1s, C 1s, O 1s and Pt 4f peaks clearly without any other
signals. The N 1s XPS results for the as-prepared Pt/CPF-C
catalyst shown in Fig. 3b, and it is deconvoluted using three
different binding energies: pyridinic-nitrogen (398.4 eV, 15.1%),
pyrrolic-nitrogen (400.0 eV, 48.1%) and graphitic-nitrogen
(401.7 eV, 36.8%). Obviously, the pyrrolic-nitrogen is domi-
nant component. As shown in Fig. 3c, the Pt 4f XPS spectra
contain two peaks corresponding to Pt 4f7/2 and 4f5/2 states from
the spin–orbital splitting. The peaks at 71.9 eV (Pt 4f7/2) and 75.2
ꢀ
450 and 680 C. Finally, some of the phenanthroline chelate is
decomposed and bounded, forming CPF-C as a nitrogen-
functionalized graphite-like structure. The total weight loss is
73.5% of the initial weight at 800 ꢀC. The structure and quality
of CPF-C were also conrmed by Raman spectroscopy (see ESI
Fig. S1†), and the results show that the CPF-C has a high
graphitization degree compared to carbon. It would be related
to the more defects and edge plane generated by the function-
alized nitrogen onto the carbon surface. The resultant CPF-C
can be an ideal catalytic support to homogeneously
eV (Pt 4f5/2) are attributed to Pt2+ specie, while 70.9 eV (Pt 4f7/2
)
and 74.2 eV (Pt 4f5/2) are accorded to Pt0 specie.37 It is known
that the nitrogen species can form a strong interaction with Pt.38
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Fig. 2 The morphology of Pt/CPF-C catalyst: (a) low-magnification TEM image; (b) size distribution histograms; (c) high-magnification TEM
image; (d) corresponded SAED pattern.
XRD measurements were performed to investigate the crystal Pt and CPF-C. Therefore, these, as well as probably a better
structures of the Pt/CPF-C and Pt/C catalysts. From the Fig. 3d, conductivity of the CPF-C support, substantially may lead to a
the rst broad peaks are attributed to the graphite structure of large ECSA value for Pt/CPF-C catalyst.41,42
(200) plane. Other remaining peaks are assigned to the (111),
MOR measurements were further performed in 0.5 M H2SO4
(200), (220) and (311) crystal facets of Pt (fcc) crystal structure. and 0.5 M CH3OH solution in Fig. 4b. The MOR peaks (about 0.7
Using the Debye–Scherrer equation relating to the (111) peak V vs. Ag/AgCl) are observed in the forward scan and anodic peaks
broadness, the mean particle size could be evaluated from the (about 0.5 V vs. Ag/AgCl) are detected during the reverse scan.
literature.39 The average particle size of the Pt/C and Pt/CPF-C is The mass activity of the Pt/CPF-C catalyst is 0.443 A mgꢂ1
,
Pt
3.6 nm and 3.8 nm, respectively. These results are consistent which is about two times higher than that of the Pt/C (0.207 A
with the results obtained from TEM. It is worth to note that, the mgꢂ1 Pt). These are consistent with the ECSA analysis. The ratio,
C (200) peak of Pt/CPF-C shis to lower 2q value compared to the forward-scan peak current (If) versus the reverse-scan peak
that of Pt/C (inserted in Fig. 3d). Additionally, the peak intensity current (Ib), is a key index for evaluating the catalyst's tolerance
is relatively decreased. These indirectly prove that carbon has to the accumulation of intermediate carbonaceous species.38
been functionalized by carbonized phenanthroline precursor. high If/Ib indicates a good poisoning tolerance of the catalyst for
A
To demonstrate the advantage of CPF-C support, the elec- MOR. The If/Ib ratio of Pt/CPF-C is 0.86, which is higher than
trochemical performance of Pt/CPF-C was compared with Pt/C. that of Pt/C (0.78). It implies the Pt/CPF-C has a better tolerance
The cyclic voltammogram (CV) curves show the general features to CO and more complete oxidation of methanol to CO2 than
of Pt. As shown in Fig. 4a, the well-dened hydrogen that of the Pt/C catalyst. Furthermore, from window inserted in
adsorption/desorption peaks and characteristic peaks for the Fig. 4b, the onset potential for the Pt/CPF-C catalyst is lower
redox process of Pt are observed on Pt/CPF-C and Pt/C catalysts. than that of Pt/C. This change indicates that the Pt/CPF-C
The electrochemically active surface area (ECSA) of catalysts was catalyst could catalyze MOR at low potential, demonstrating
evaluated by the hydrogen adsorption/desorption of Pt accord- that the Pt/CPF-C has desirable kinetics for MOR.
ing to literature.40 The Pt/CPF-C shows a larger ECSA value (86.3
Long-term stability and durability of catalyst are critical
m2 gꢂ1Pt) than that of the Pt/C (45.7 m2 gꢂ1Pt). The nitrogen- requirement for electrochemical energy device applications.
functionalized structure of CPF-C support can modify nucle- MOR cycling and chronoamperometry (CA) were carried out to
ation and growth kinetics during the deposition of Pt nano- examine the stability and durability of the Pt/CPF-C and Pt/C
particles, which results in high dispersion, makes the support catalysts. As can be seen in Fig. 4c, based on Pt mass, the Pt/
surface chemically active for the enhanced interaction between CPF-C catalyst loses 58.8% of MOR activity aer 500 cycles,
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Fig. 3 XPS spectrum of Pt/CPF-C (a); N 1s XPS spectrum of Pt/CPF-C (b); Pt 4f XPS spectrum of Pt/CPF-C (c); XRD patterns of Pt/CPF-C and Pt/C
(d). Insert: enlarged C (200) peaks.
whereas that of the Pt/C catalyst has decreased by 64.0%. where I0 is the current at the start of polarisation back extrap-
During the electrocatalytic processes, the increased Pt particle olated from the linear current decay, (dI/dt)t>500 s is the slope of
size, the aggregation of Pt particles, and the falling off of Pt the linear portion of the current decay. The calculated d values
particles from the support are considered to be the major of Pt/CPF-C and Pt/C are 0.021 and 0.288, respectively. Clearly,
reasons for the Pt MOR activity loss.43 Obviously, the rate of the the strong interaction between Pt nanoparticles and CPF-C
degradation of the Pt/CPF-C catalyst is relatively slower than substrate leads to relatively improved stability and durability
that of the Pt/C catalyst. This is considered that the nitrogen- of the Pt/CPF-C catalyst. These are also consistent with MOR
functionalized support in Pt/CPF-C catalyst makes the Pt cycling tests in Fig. 4c.
particles less vulnerable to dissolute and aggregate than those
Fig. 5a shows a typical linear sweep voltammogram (LSV) at
supported on the carbon, leading to a better stability for MOR. the scan rate of 1 mV sꢂ1 in 0.5 M H2SO4 + 0.5 M CH3OH
Although the complete oxidation of methanol yields CO2 as solution, which is close to a steady-state polarization curve of
the nal product, the complete oxidation seldom takes place. MOR.45 Obviously, the current of the Pt/CPF-C catalyst remains
There are numerous intermediate products, including adsorbed higher in contrast with the Pt/C catalyst throughout the test,
poisons, formed during MOR. Therefore, the decay of current which is similar to the results of Fig. 4b above. The corre-
implies the poisoning of the catalyst. CA measurements of Pt/ sponded Tafel plots of the MOR are shown in Fig. 5b, calculated
CPF-C and Pt/C were performed at 0.6 V (vs. Ag/AgCl) for 3000 from the quasi-steady-state curves in Fig. 5a. It shows two linear
s. As shown in Fig. 4d, the initial current density for Pt/CPF-C is regions with the respective Tafel slope. The Tafel slope of the Pt/
greater than that of the Pt/C, implying a greater number of CPF-C is 116.3 mV decꢂ1 (<ꢂ0.45 V vs. Ag/AgCl), and it turns
active sites available per unit of surface area. Obviously, the Pt/ into 194.4 mV decꢂ1 (>ꢂ0.45 V vs. Ag/AgCl). While, Pt/C is 111.8
CPF-C has a higher current density than that of Pt/C over the mV decꢂ1 (<ꢂ0.42 V vs. Ag/AgCl), and it turns into 238.9 mV
whole process, showing that it has a higher poison-tolerance. decꢂ1 (>ꢂ0.42 V vs. Ag/AgCl). As the potential increasing, two
The linear decay of the current at times greater than 500 s Tafel slopes from low values increases up to high values. It
may be characterised by the long-term poisoning rate (d) as indicates the possible changes of rate-determining step and
follows:44
mechanism within a given potential range. It has been reported
that the splitting of the rst C–H bond of methanol molecules
d ¼ (100/I0) ꢃ (dI/dt)t>500 s (% sꢂ1
)
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Fig. 4 CV of Pt/CPF-C and Pt/C in 0.5 M H2SO4 solution. Scan rate: 50 mV sꢂ1 (a); MOR of Pt/CPF-C and Pt/C in 0.5 M H2SO4 + 0.5 M CH3OH
solution. Scan rate: 50 mV sꢂ1 (b). Inserted: enlarged window from 0.1 V–0.3 V; decreases of peak currents of Pt/CPF-C and Pt/C on cycling in 0.5
M H2SO4 + 0.5 M CH3OH (c); CA of Pt/CPF-C and Pt/C in 0.5 M H2SO4 + 0.5 M CH3OH (d). Inserted: current after 3000 s; fixed potential: 0.6 V.
and mass transfer of methanol are the rate-determining step at vertical increase of the reverse oxidation peaks (about 0.5 V vs.
relatively low and high potentials, respectively.46,47
Ag/AgCl), which are more prominent at higher temperatures.
The effect of temperature on MOR was also investigated. Similar to the literature,48 these indicate the residues remaining
Fig. 6a and c show the MOR on the Pt/CPF-C and Pt/C catalysts unoxidized on the surface of catalyst during the forward sweep
at different temperature values, respectively. Obviously, current can only be oxidized at higher potentials.
density for MOR increases with the temperatures rising. Addi-
The comparison of activation energy values can give insight
tionally, the MOR current obtained in the forward sweep (about into the performance difference between two catalysts in MOR.
0.65 V vs. Ag/AgCl) is raised by a factor of 6 as the temperature The value of the activation energy can be obtained from the
increasing from 278 to 308 K, while the negative sweeps display slope of the linear relationship between log j and 1/T. As can be
Fig. 5 LSV of Pt/CPF-C and Pt/C in 0.5 M H2SO4 + 0.5 M CH3OH solution. Scan rate: 1 mV sꢂ1 (a); the corresponded Tafel plots of Pt/CPF-C and
Pt/C (b).
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Fig. 6 MOR tests on different temperatures for Pt/CPF-C (a) and Pt/C (c) in 0.5 M H2SO4 + 0.5 M CH3OH solution. Scan rate: 50 mV sꢂ1; the
corresponded Arrhenius plots of Pt/CPF-C (b) and Pt/C (d).
seen in Fig. 6b and d, well-linear relation between log j and 1/T is Innovative Research Team in University (IRT1177), Key Labo-
obtained in the temperature range from 278 to 308 K. And the ratory of Eco-Environment-Related Polymer Materials of
average apparent activation energy has been determined. The Ministry of Education and Key Laboratory of Polymer Materials
lower activation energy value of 17.51 kJ molꢂ1 on Pt/CPF-C for of Gansu Province.
MOR is obtained against 17.61 kJ molꢂ1 on Pt/C. A slight low
activation energy also means high intrinsic activity and fast charge-
transfer process. In other words, any reduction in the activation
References
energy would be translated into the high intrinsic activity. It is
reasonable to deduce that nitrogen-functionalized CPF-C support
is benecial for Pt/CPF-C catalyst towards MOR in DMFCs.
1 S. Guo, S. Zhang, D. Su and S. Sun, J. Am. Chem. Soc., 2013,
135, 13879.
2 P. Malacrida, M. Escudero-Escribano, A. Verdaguer-
Casadevall, I. E. L. Stephens and I. Chorkendorff, J. Mater.
Chem. A, 2014, 2, 4234.
3 L. Wang and Y. Yamauchi, J. Am. Chem. Soc., 2013, 135,
16762.
4 S. I. Choi, S. Xie, M. Shao, J. H. Odell, N. Lu, H. C. Peng,
L. Protsailo, S. Guerrero, J. Park, X. Xia, J. Wang, M. J. Kim
and Y. Xia, Nano Lett., 2013, 13, 3420.
5 S. Xie, S. I. Choi, N. Lu, L. T. Roling, J. A. Herron, L. Zhang,
J. Park, J. Wang, M. J. Kim, Z. Xie, M. Mavrikakis and Y. Xia,
Nano Lett., 2014, 14, 3570.
6 E. Antolini, Mater. Chem. Phys., 2003, 78, 563.
7 S. Mukerjee and R. C. Urian, Electrochim. Acta, 2002, 47,
3219.
4. Conclusion
In this study, a carbonized phenanthroline functionalized
carbon support (CPF-C) has been prepared to load Pt nano-
particles towards MOR. Compared to commercial Pt/C catalyst,
the as-synthesized Pt/CPF-C catalyst shows relatively high
performance, with respect to onset potential, mass activity,
stability and durability. The temperature investigation also
shows that the Pt/CPF-C catalyst has a relatively low activation
energy value for MOR. Consequently, the CPF-C can be used as a
promising alternative support to intensify Pt applied in DMFCs.
8 X. Zhao, M. Yin, L. Ma, L. Liang, C. Liu, J. Liao, T. Lu and
W. Xing, Energy Environ. Sci., 2011, 4, 2736.
9 Y. Takasu, H. Itaya, T. Iwazaki, R. Miyoshi, T. Ohnuma,
W. Sugimoto and Y. Murakami, Chem. Commun., 2001, 341.
Acknowledgements
We thank to the National Natural Science Foundation of China
(no. 21174114), the program for Changjiang Scholars and
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 17216–17222 | 17221

View Article Online
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Paper
10 Z. Sun, Y. Zhao, Y. Xie, R. Tao, H. Zhang, C. Huang and 29 D. W. Wang, I. R. Gentle and G. Q. Lu, Electrochem.
Z. Liu, Green Chem., 2010, 12, 1007. Commun., 2010, 12, 1423.
11 B. Sheng, L. Hu, T. Yu, X. Cao and H. Gu, RSC Adv., 2012, 2, 30 Z. Li, J. Liu, C. Xia and F. Li, ACS Catal., 2013, 3, 2440.
5520.
31 D. Liu, L. Yang, J. S. Huang, Q. H. Guo and T. Y. You, RSC
Adv., 2014, 4, 13733.
32 J. N. Tiwari, K. C. Kemp, K. Nath, R. N. Tiwari, H. G. Nam and
K. S. Kim, ACS Nano, 2013, 7, 9223.
33 T. Zhou, H. Wang, J. Key, S. Ji, V. Linkov and R. Wang, RSC
Adv., 2013, 3, 16949.
34 J. Cao, Y. Chu and X. Tan, Mater. Chem. Phys., 2014, 144, 17.
12 C. Nethravathi, E. A. Anumol, M. Rajamathi and
N. Ravishankar, Nanoscale, 2011, 3, 569.
13 Y. Mu, H. Liang, J. Hu, L. Jiang and L. Wan, J. Phys. Chem. B,
2005, 109, 22212.
14 Y. Li, W. Gao, L. Ci, C. Wang and P. M. Ajayan, Carbon, 2010,
48, 1124.
15 W. X. Chen, J. Y. Lee and Z. Liu, Chem. Commun., 2002, 2588. 35 Z. Liu, F. Su, X. Zhang and S. W. Tay, ACS Appl. Mater.
16 Y. C. Kimmel, X. Xu, W. Yu, X. Yang and J. G. Chen, ACS
Catal., 2014, 4, 1558.
Interfaces, 2011, 3, 3824.
36 Y. Wei, Z. Zhao, J. Liu, C. Xu, G. Jiang and A. Duan, Small,
2013, 9, 3957.
´
˜
17 M. Roca-Ayats, G. Garcıa, J. L. Galante, M. A. Pena and
´
M. V. Martınez-Huerta, J. Phys. Chem. C, 2013, 117, 20769.
37 H. H. Li, S. Zhao, M. Gong, C. H. Cui, D. He, H. W. Liang,
L. Wu and S. H. Yu, Angew. Chem., Int. Ed., 2013, 52, 7472.
18 G. Agostini, C. Lamberti, R. Pellegrini, G. Leofanti,
F. Giannici, A. Longo and E. Groppo, ACS Catal., 2014, 4, 187. 38 H. Huang, S. Yang, R. Vajtai, X. Wang and P. M. Ajayan, Adv.
19 K. An, S. Alayoglu, N. Musselwhite, S. Plamthottam, Mater., 2014, 26, 5160.
G. Melaet, A. E. Lindeman and G. A. Somorjai, J. Am. 39 V. I. Zaikovskii, K. S. Nagabhushana, V. V. Kriventsov,
Chem. Soc., 2013, 135, 16689. K. N. Loponov, S. V. Cherepanova, R. I. Kvon,
´
`
¨
20 C. Alegre, M. Galvez, R. Moliner, V. Baglio, A. Arico and
H. Bonnemann, D. I. Kochubey and E. R. Savinova, J. Phys.
´
M. Lazaro, Appl. Catal., B, 2014, 147, 947.
Chem. B, 2006, 110, 6881.
21 J. Liang, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. 40 J. Zhao, H. Yu, Z. Liu, M. Ji, L. Zhang and G. Sun, J. Phys.
Ed., 2012, 51, 11496. Chem. C, 2014, 118, 1182.
22 D. Higgins, M. A. Hoque, M. H. Seo, R. Wang, F. Hassan, 41 G. Hu, F. Nitze, T. Shari, H. R. Barzegar and T. Wagberg, J.
J. Y. Choi, M. Pritzker, A. Yu, J. Zhang and Z. Chen, Adv.
Funct. Mater., 2014, 24, 4325.
Mater. Chem., 2012, 22, 8541.
42 Y. W. Lee, M. Kim and S. W. Han, Chem. Commun., 2010, 46,
23 Z. Liu, J. Y. Lee, W. Chen, M. Han and L. M. Gan, Langmuir,
2004, 20, 181.
1535.
43 C. Zhang, L. Xu, N. Shan, T. Sun, J. Chen and Y. Yan, ACS
24 M. Carmo, V. A. Paganin, J. M. Rosolen and E. R. Gonzalez, J.
Power Sources, 2005, 142, 169.
Catal., 2014, 4, 1926.
44 J. Jiang and A. Kucernak, J. Electroanal. Chem., 2003, 543,
25 G. Hu, F. Nitze, H. R. Barzegar, T. Shari, A. Mikołajczuk,
187.
˚
C. W. Tai, A. Borodzinski and T. Wagberg, J. Power Sources, 45 S. Cavaliere, S. Subianto, I. Savych, M. Tillard, D. J. Jones and
`
2012, 209, 236.
J. Roziere, J. Phys. Chem. C, 2013, 117, 18298.
26 M. K. Debe, Nature, 2012, 486, 43.
46 G. Wu, L. Li and B. Q. Xu, Electrochim. Acta, 2004, 50, 1.
˚
27 E. Gracia-Espino, X. Jia and T. Wagberg, J. Phys. Chem. C, 47 H. Wang, R. Wang, H. Li, Q. Wang, J. Kang and Z. Lei, Int. J.
2014, 118, 2804. Hydrogen Energy, 2011, 36, 839.
28 N. I. Kim, J. Y. Cheon, J. H. Kim, J. Seong, J. Y. Park, S. H. Joo 48 S. S. Mahapatra, A. Dutta and J. Datta, Electrochim. Acta,
and K. Kwon, Carbon, 2014, 72, 354. 2010, 55, 9097.
17222 | RSC Adv., 2015, 5, 17216–17222
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